A diffraction grating is an arrangement composed of a multiplicity of reflecting or transmitting diffraction structures which follow one another periodically at the spacing of a grating period (d). The diffraction structures can be light-transmitting slits or openings in a screen, or an aggregation of reflecting channels or grooves on a substrate. Light which falls onto a diffraction grating is diffracted at the diffraction grating.
A class of optical diffraction gratings which are termed echelle gratings, or simply echelles. Each individual diffraction structure in the form of a grating flute or grating line has a profile which consists of two edges, a short edge, which is also termed a blaze flank, and a long flank, which is also termed an antiblaze flank. The blaze flank and the antiblaze flank form together with the base surface a triangular profile, which is also termed a blaze profile.
An optical arrangement provided with the general reference numeral 100 and having a diffraction grating 102 is illustrated for explanatory purposes in FIG. 1. Two diffraction structures 104 of the multiplicity of diffraction structures are illustrated from the diffraction grating 102. A base surface of the diffraction grating 102 is provided with the reference numeral 106. The diffraction structures 104 each have a blaze flank 108 and an antiblaze flank 110.
Each blaze flank 108 is inclined at an angle β to the base surface 106, and each antiblaze flank 110 is inclined at an angle α to the base surface 106. Each blaze flank 108 encloses a so-called apex angle γ with the neighbouring antiblaze flank 110. It holds for the apex angle γ that: γ=180°−α−β.
When a light beam 112 with light of a wavelength λ is incident on the diffraction grating 102, the light is diffracted in many discrete directions, these directions being termed diffraction orders. The number of the diffraction orders is finite and is governed by the ratio of the grating period d of the diffraction grating to the wavelength λ of the incident light and is, moreover, dependent on the incidence angle θe at which the light beam 112 falls onto the diffraction grating 102. The directions θm of the diffraction orders m are described by the following grating equation, m being the number of the respective diffraction order:
                              m          ·                      λ            d                          =                              (                                          sin                ⁢                                                                  ⁢                                  θ                  e                                            +                              sin                ⁢                                                                  ⁢                                  θ                  m                                                      )                    .                                    (        1        )            
The angles θe and θm are measured in relation to the grating normal GN, which is perpendicular to the base surface 106 in FIG. 1.
Echelle gratings are usually operated in a high diffraction order m and at a large diffraction angle θm. Common diffraction angles θm lie mostly between 63° and 85°. Used diffraction orders m frequently lie between 30 and 150, it also being possible for extreme diffraction orders m to lie at about 600. Advantageous properties of echelle gratings are a high angular dispersion and a high resolution, as well as a relatively high diffraction efficiency over a large spectral range, and being relatively free from polarization effects.
Diffraction efficiency in a specific diffraction order m is understood as the ratio of the intensity of the light diffracted in this diffraction order m to the intensity of the incident light, and can correspondingly be at most 1.0 or 100%, assuming that no absorption occurs in the diffraction grating. However, in a real case, because of absorption during the diffraction at metal gratings, the sum of all the diffraction efficiencies is always smaller than 100%, while it can be 100% only in the ideal case of an ideally conducting material of infinite conductivity. The same holds in the case of a transmissive diffraction grating when the material is purely transmissive.
Moreover, in the case of the diffraction orders m a distinction is made between the so-called propagating diffraction orders and the non-propagating diffraction orders. Non-propagating diffraction orders are invisible and are also sometimes termed as “lying below the horizon”. The grating equation (1) describes the propagating diffraction orders.
As the diffraction efficiency in the individual diffraction orders is very different, an attempt is made in many applications to direct as much light as possible into a single diffraction order. This effect is denoted as a blaze effect, and the corresponding diffraction order is termed a blaze order. Neighbouring diffraction orders of the blaze order can in this case still include a considerable fraction of the diffracted light.
In some applications, it is the so-called Littrow arrangement that is selected for the optical arrangement which includes the diffraction grating. As illustrated in FIG. 1, in the Littrow arrangement the diffraction grating is orientated to the incident light beam 112 such that the emergent light beam 114 of blaze order m is diffracted back in the same direction from which the incident light beam 112 comes. It holds in this case that θe=θm=θL, with θL being denoted as the Littrow angle.
In the document U.S. Pat. No. 6,067,197, the diffraction grating is orientated in a Littrow arrangement. The diffraction structures of the diffraction grating are configured in the case of the known optical arrangement such that the blaze flanks of the grating are struck by the incident light beam virtually perpendicularly, that is to say it holds that θL=β, that is to say Littrow angle θL and blaze angle β of the blaze flanks are virtually equal.
The grating equation (1) is simplified to
                              sin          ⁢                                          ⁢                      θ            L                          =                  m          ⁢                      λ                          2              ⁢                                                          ⁢              d                                                          (        2        )            in the Littrow arrangement in which θL=θe=θm.
In the case of a grating configuration in which the blaze order is the last propagating diffraction order, it emerges that this blaze order has a somewhat higher diffraction efficiency for echelle gratings in a Littrow arrangement than a blaze order in another diffraction order. There are also blaze arrangements in which after the blaze order there is still at least one further diffraction order which has a larger number and which is therefore diffracted at a larger angle than the blaze order.
In the case of the optical arrangement in accordance with U.S. Pat. No. 6,067,197, the grating period d of the diffraction grating and the Littrow angle θL can be matched such that, as it were, a high blaze action is reached for two wavelengths λ1, λ2 by operating the diffraction grating for two wavelengths λ1, λ2 in two different last propagating blaze orders m1, m2. As already mentioned, in the case of this known arrangement the Littrow angle θL is virtually equal to the angle β of the blaze flanks to the base surface of the diffraction grating.
A further optical arrangement including a diffraction grating is known from the document U.S. Pat. No. 6,762,881; here, the optical wavelength λ, the grating period d and the Littrow angle θL are matched such that use is made for light retroreflected in the Littrow angle θL of the diffraction grating in one of the largest diffraction orders m which still fulfils the condition (2((m+1)/m)−1)sin θL≧1. An aim of this known arrangement is to achieve an increased diffraction efficiency.
A further detailed description of diffraction gratings, in particular echelle gratings, is to be found in the technical book by Erwin G. Loewen and Evgeny Popov entitled “Diffraction Gratings and Applications”, Marcel Dekker Inc., New York, 1997. It is defined there that the blaze effect, that is to say the property of diffraction gratings to concentrate the diffracted light in a specific diffraction order, is perfect when no light goes in another direction than in the blaze order, the absolute diffraction efficiency being limited only by absorption losses and diffuse scattering. Although the production of diffraction gratings continued to be improved yet further, some diffraction gratings do not, however, display a perfect blaze effect, that is to say in addition to the diffracted light of the desired blaze order, whose intensity has a maximum, light is also always to be found in further diffraction orders which therefore subtract intensity from the desired blaze order.
A theorem which attempts to explain the conditions under which there can be a perfect blaze effect was formulated to this end in a paper by A. Marechal and G. W. Stroke entitled “Sur l'origine des effets de polarisation et de diffraction dans les réseaux optiques” [“On the origin of the effects of polarization and of diffraction in optical gratings”], C. R. Ac. Sc. 249, 2042-2044 (1980). According thereto, it is possible very generally for a grating with a plurality of diffraction orders to exhibit an optimal blaze effect in a blaze order when the diffraction grating profile is a triangular profile with an apex angle γ of 90°. The abovementioned paper then also specified physical reasons for this selection for the case of ideal conductivity and in TM polarization of the light. It is possible to substantiate for this case that no light goes into other diffraction orders and that, therefore, a perfect blaze effect is present. A reflecting material with ideal conductivity reflects incident light at 100% and therefore has no losses which could extract energy from the incident light. However, the theorem from the paper loses its validity for TE polarization, in the case of which other conditions placed on the electromagnetic optical field are present than in the case of TM polarization.
In the abovenamed technical book the conclusion is drawn therefrom that a perfect blaze effect in the TE polarization can exist for other incidence angles, but that the diffraction efficiency in the TE polarization can never be 100% when a perfect blaze effect occurs in the TM polarization.
In certain known optical arrangements and known diffraction gratings, the blaze effect can depend relatively strongly on the accuracy of manufacturing of the diffraction structures. In instances, the angle β of the blaze flanks relative to the base surface of the diffraction grating lies in a very narrow specification and is allowed to have at most a deviation of approximately 0.5° in relation to an optimum angle β′. An overshooting of this angular tolerance can lead to a steep drop in the efficiency of response of the diffraction grating in the selected configuration.